This invention relates to an image reading apparatus for reading out a document such as a book or a magazine, and a method for processing read out image data thereof.
Conventionally, an image reading apparatus known as a book scanner is provided. In such an apparatus, a pickup unit is arranged above a document table, and an opened space whose height is several dozen centimeters, exists between the document table and the pickup unit. When reading out bound documents such as a book or a magazine (hereinafter referred to as "bookform document"), a user places a bookform document which is opened with face up on the document table. Scanning for a document surface (a surface to be read out) is started in response to that a start key is ON, and a read out image is output to an external device one by one simultaneously with the scanning. If the external device is a printer, a copied image is provided in real time.
In the above-mentioned book scanner, the user can turn pages on the document table, so it is possible to raise efficiency for reading out many pages. Moreover, compared with a case in which a document is placed face down, damage to the document is far more reduced. Besides, the bookform documents also include filed documents.
Unlike a sheet-form document, a document surface of the bookform document is curved. Therefore, a distortion is caused in a read out image according to the curvature condition. The distortion is rectified, and an image having an equal picture quality to that in case of reading out a sheet form document, is output to the external device. As shown in FIG. 8, the distortion rectification is a variable magnification procedure in which each line (a group of pixels which line up in a perpendicular direction to the direction M2 in which the document is opened) of the read out image is duplicated in accordance with a varied amount of the document height (an inclination) of the document surface S1. By means of the distortion rectification, a length x of the read out image in the direction M2 is elongated into a length x' that is a length when the document surface S1 is supposed to be plane. As for a method of detecting changes of document height in the document surface S1, there are a method of executing an image analysis by photographing an upper end surface (so-called "head" in books) of a document (refer to Japanese Patent Unexamined Publication No. 5-161002), and a slit light projection optical method which is employed for a three-dimensional measuring (refer to Japanese Patent Unexamined Publication No. 5-219323).
The book scanner is constructed so as to execute binarization for a readout image whose distortion is rectified as mentioned above by a method of reproducing medium gradations, such as dither method, and output the binarized image to an external device.
FIG. 9 is a view showing a data processing method of a conventional book scanner. Conventionally, a distortion rectification and binarization are executed in sequence. For example, 4-bit (16 gradations) read out data D1 are output from an analog/digital (A/D) converter (not shown) to a distortion rectification unit 91 in their scanned order (order of pixel arrangement). The distortion rectification unit 91 consists of a memory 911 for a prescribed capacity, a write-in control unit 912, and a readout control unit 913. The write-in control unit 912 writes the read out data D1 in the memory 911 in input order. In accordance with an instruction of a controller which is not shown, the read out control unit 913 reads out image data of each line from the memory 911 for the number of times corresponding to the duplicated number of each line (in case that the line is not duplicated, it is counted as "1"), and transmits it to a binarization unit 92. For instance, in case that one line for line j corresponds to 2 lines after the rectification (the duplicated number is "2"), it is read out two times. That is, the first time readout for line j is executed simultaneously with the write-in for line (J+1), and the second time readout for line j is executed simultaneously with the write-in for line (j+2). After that, line (j+1) is read out. A distortion caused by a curvature in the document surface is rectified by selecting the number of times of reading out for every line, as mentioned here.
The binarization unit 92 consists of a dither-matrix 921 of 4 lines and 4 rows, a selector 922, and a comparator 923. A selector 922 selects each line of the dither-matrix 921 one by one when a line synchronizing signal is input, and transmits the selected 4 threshold values to the comparator 923. The comparator 923, cyclically applying the 4 threshold values, converts the read out data D1 into binarized (1 bit) read out data D2.
In the above-mentioned distortion rectification, even if address spaces are cyclically utilized by simultaneously executing the write-in and readout in the memory 911, it needs at least a memory capacity for lines corresponding to the difference between lines before the rectification and lines after the rectification. For example, in case of reading out a bookform document that is A3 size in its opened state, and its height difference in the document surface is 50 mm in a resolution of 400 dpi, data on 1000 lines have to be temporarily memorized.
Conventionally, when the number of pixels in a line is 5000, and the number of gradations for reading out is 16 (4 bits), the memory 911 is needed to have a large capacity of 20 M bits (=4.times.5000.times.1000 bits). In case that the number of gradations is 256 (8 bits), a needed memory capacity is 40 M bits.
If the distortion rectification is executed after binarization, the memory capacity is able to be reduced. However, in such a case, notches in an outline of the image become conspicuous, so its picture quality is lowered.